Due to cool water temperatures (about 40° F. (4.4° C.)) in deep water offshore hydrocarbon recovery operations, hydrocarbon fluids flowing through subsea pipelines become very viscous or deposit paraffin when the temperature of the fluid drops, adversely affecting fluid flow in the pipeline. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a “hydrate”. Hydrates can plug pipelines and the plugs may be very difficult to remove.
One solution involves electrical heating of the subsea pipeline to prevent excessive cooling of the fluid hydrocarbons. Heating by a variety of electrical methods has been known. Two configurations for electrical heating have been considered. One configuration, called a Single Heated Insulated Pipe (SHIP) system uses a single, electrically insulated flowline with current passing along the flowline. Another configuration is called a pipe-in-pipe system (EHPIP).
An EHPIP subsea pipeline has a flow line or inner pipe for transporting well fluids which is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage is applied between the inner and outer pipes at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, which is hereby incorporated by reference. U.S. Pat. Nos. 6,161,025, 6,179,523, 6,264,401, 6,292,627, 6,315,497, 6,371,693; and commonly owned patent applications titled “Annulus for Electrically Heated Pipe-in-Pipe Pipeline”, Ser. No. 09/910,696; “Method of Installation of Electrically Heated Pipe-in-Pipe Subsea Pipeline”, Ser. No. 09/910,678, Publication No. US2003/0017007A1; “Method for Commissioning and Operating an Electrically Heated Pipe-in-Pipe Subsea Pipeline”, Ser. No. 09/910,622, Publication No. US2003/0020499A1; “Corrosion Protection of Electrically Heated Pipe-in-Pipe Subsea Pipeline”, Ser. No. 09/910,489, Publication No. US2003/0015436A1; “Power Supply for Electrically Heated Subsea Pipeline”, Ser. No. 09/910,625, Publication No. US2003/0015519A1; “Apparatus and Method for Electrically Testing of Electrically Heated Pipe-in-Pipe Pipeline”, Ser. No. 09/910,295, Publication No. US2003/0016028A1, are all hereby incorporated by reference.
Referring to FIG. 1, the general concept a pipe-in-pipe heating segment 10 is illustrated. Flow pipe 12 is positioned concentrically within outer pipe 14, so that annulus 13 is defined between the pipes. Concentric pipes 12 and 14 are electrically isolated except at bulkheads 16, which are placed at each end of the selected segment 10 of pipeline to be heated. Electrical power supply 6 supplies voltage at a selected voltage and frequency (including Direct Current) between flow pipe 12 and outer pipe 14 to a selected point on the pipes. Typically, the electrical voltage is supplied to the heating segment 10 at the mid-point between bulkheads 16. However, voltage may also be supplied at a location offset from the mid-point between bulkheads 16, such that a difference in electrical impedance between each portion of the segment to be heated is taken into account (for example, to allow equal current flow in each portion of the heated segment, even though impedance is different) or to provide more power for heating in one portion of the selected segment. While voltage may be supplied anywhere, in this disclosure an electrical connector between bulkheads will be referred to as a “mid-line connector”. Adjoining heating segments may be electrically heated. A single bulkhead between two heating sections may complete two electrical circuits, such that electrical current from both segments passes through the single bulkhead. The length of a heating segment may be from a few feet, for example about 50 feet (15.2 m), to 40 miles (64.4 km) or more, depending on the requirements for heating the pipeline. More typically, the length of heating segments range from about 1 mile (1.6 km) to about 10 miles (16.1 km).
Referring to FIG. 2, a system implementing a mid-line pipe-in-pipe electrically heated system is shown. A pipeline 2 is deployed normally thousands of feet below sea surface 1 having a first end near platform 3, a floating facility or other host facility, and having a second end on the sea floor 4. Platform 3 is anchored to sea floor 4. Riser 5 connects heating segment 10 to the top side of platform 3 or other facility. Riser 5 may also be heated using the pipe-in-pipe configuration, in which case it will be treated as a heating segment 10 of the pipeline 2. Electrical generator 6 is supported on platform 3, among other equipment. Electrical generator 6 is connected electrically by cable 7 to mid-line connectors 20. Heating segments 20 are separated by bulkhead 16.
In typical pipe-in-pipe methods of heating, the total voltage drop is maintained at the power supply-end of the pipe segment to be heated. The voltage drop at the power input end of a heated segment determines the amount of heating available and the length of a segment that can be heated. Voltage drop is limited by the dielectric strength and thickness of electrical insulation available. A configuration for minimizing voltage required with the pipe-in-pipe method is needed. Also, there is a need for an apparatus and method that allow heating selected segments of a pipeline that is heated by the pipe-in-pipe method.
A pipe-in-pipe mid-line connector is needed which may be connected to the electrical cable after the pipe-in-pipe is deployed to the sea floor. A pipe-in-pipe mid-line connector is also needed which provides relative axial movement between the flow pipe and the outer pipe to avoid structural failures. During deployment, the pipe-in-pipe pipeline must endure bending stresses and tensile stresses which are significant. A mid-line connector is needed which enables flexibility between the flow pipe and the outer pipe but also enables both the flow pipe and the outer pipe to share the tensile loads. A mid-line connector which reduces local heating caused by poor contact resistance is also needed. Further, there is a need for a mid-line connector which protects the flow pipe from local corrosion and build-up of oxidation particles within the mid-line connector. Because mid-line connector may be electrically unused for long periods of time, a mid-line connector which will not allow its electrical contacts to become corroded is needed. Because mid-line connectors are the electrical power supply points for pipeline heating segments, a mid-line connector which controls stray current effects on the mid-line connector is needed. Still further, a pipe-in-pipe system is needed which immobilizes intra pipe movement between the flow pipe and the outer pipe.